rock scissors paper, played for keeps

[Laurence Horn]

Recalling our colloquy from July 2003, but in a different key.  This
appears in full in The Scientist online, at
http://www.the-scientist.com/2005/4/11/32/1, but I think you'll need
to subscribe (for free) to get the link.  I was not aware that the
motif, and term, can be invoked on an evolutionary scale.

Larry
(hoping that this will be readable despite my fears that the lines
will end up all skewed)
=====================


  Volume 19 | Issue 7 | 32 | Apr. 11, 2005
  Getting by in a Game without Winners

Is "rock-paper-scissors" a common biological motif?
by Stuart Blackman

       A time-honored tradition for choosing teams, riding shotgun,
and settling other childish disputes, the game called
rock-paper-scissors has been around far longer than humans have been
playing it.

       A time-honored tradition for choosing teams, riding shotgun,
and settling other childish disputes, the game called
rock-paper-scissors has been around far longer than humans have been
playing it. Similar nontransitive games, in which no one strategy
reigns over all others, are played out among certain lizards,
microbes, and marine organisms. And some biologists are suggesting
that, rather than being a mere biological oddity, the
rock-paper-scissors dynamic is a widespread phenomenon that maintains
genetic diversity within species and ecosystems.

       "If the environment underlying the system is homogeneous,
intuition would suggest there's going to be a good competitor that
drives out all the others," says Ben Kerr, research associate at the
University of Minnesota. "In rock-paper-scissors, the system itself
has all the cogs and gears to generate diversity."

       MATING GAMES
In California, rock-paper-scissors is played out between three male
morphs of the side-blotched lizard, which are distinguished by their
throat colors. Big, testosterone-pumped, orange-throated males beat
up on their smaller, blue-throated rivals, who in turn dominate the
smallest, yellow morph. So far, so transitive. But, by mimicking the
female of the species, yellows escape the attention of oranges and
get to poach the real females attracted to the oranges' spacious
territories.

       "Selection [tends] to push a species towards one nice little
tidy optimum," says Barry Sinervo, professor of biology at the
University of California, Santa Cruz, who described the lizard's
nontransitive competitive dynamic in 1996.1 But the genetic,
behavioral, and morphologic differences among the three morphs are on
a par with those found between species, he says. Rock-paper-scissors
"takes a species and stretches it out."

       Kerr works on a similar game played out on the microscopic
scale between strains of Escherichia coli living in rodent
intestines. Certain strains produce toxins called colicins, which
they release explosively, killing sensitive competing strains while
sparing resistant clonal relatives. A third strain produces no
colicins, but is resistant to those produced by others.

       The respective costs associated with colicin resistance and
production result in a linear growth hierarchy: In the absence of
competitors, sensitive strains grow faster than resistant ones, and
these outgrow producers. But in a mixed population, toxin producers
loop the hierarchy by killing off
those at the top.

       Just as playing scissors would be the optimal strategy in the
human version of the game if most of the competition played paper,
the selective advantage to each bacterial strain or male lizard morph
depends on the relative frequencies of the other two strategies. The
result is a cycling in
the densities of the three strategies...